The present invention describes improvements to the weld selector stations described in the above U.S. Patent and the above U.S. patent application. The improvements are also applicable to other pulse arc type electronic welders.
In most welding applications, it is necessary to start the arc between a welding torch, electrode, or a rod and the workpiece by bringing the rod into very close proximity to, or into contact with, the workpiece. When an arc is started by contact or near contact with the workpiece, the initial arc content can be very high and, if over-current protection is provided, can trip the over-current protector and shut down the welder, thus necessitating restarting the arc, or, if over-current protection is not provided, the initial arc currents may be high enough to damage the welding unit or the workpiece.
Welding machines have been created for starting the arc without rod/workpiece contact or near contact. For example, see U.S. Pat. No. 4,672,175, dated June 9, 1987, issued to Robert W. Niven, and entitled "Arc Starter For Welder". However, most electronic welders do not use a high voltage arc starting apparatus and the output voltage which can be used for starting the arc is typically limited to the D.C. power supply voltage, which is generally about 80 volts.
Furthermore, in electronic pulse arc type welders, the output voltage is not continuous but is pulsed so that the average output voltage is substantially less than the D.C. power supply voltage. This lower average output voltage increases the difficulty incurred in properly starting the arc. Therefore, there is a need for a pulse arc type welder which provides an increased average output voltage during arc striking conditions.
Furthermore, at the instant when the arc is ignited, especially in those circumstances where the rod was initially touching the workpiece, the small arc gap will allow an extremely large arc current to flow if some means of protection is not provided. In some cases, current sensing circuitry is provided which interrupts the output if the peak output current exceeds a predetermined value. This has the advantage of protecting the drive circuits. However, this has the disadvantage that, if the arc is interrupted too long, the arc will be quenched and it will be necessary to reexecute the arc striking operation. In other types of systems, the average output current is measured and used to provide feedback to pulse width modulation circuitry so that the large starting current causes a reduction in the width of the output pulses. However, this type of feedback circuitry is generally designed for maintaining an arc once it has been created, has a very limited range of control over the pulse widths, and generally provides inadequate control over the starting arc current.
Therefore, there is a need in the welding art for an arc starting circuit for an electronic pulse arc type welder which provides an increased output voltage during arc striking conditions. Furthermore, there is a need in the welding art to provide an arc starter circuit which provides for the control and limiting of the initial arc starting surge current.